Dont you think this engine will be enough?
A 38cc engine would probably be too much for my style of flying on a 14-pound airplane. Cannot make a judgment about your preferences or the GeeBee. On a 14-pound airframe you might want to closely watch and occasionally inspect the structure especially at the engine mounting and firewall. My general rule is two times the weight in displacement but maybe not for a GeeBee.
A GeeBee is a special brand of unforgiving racer. It probably always must fly on the engine and may glide like a piano. It may not even fly at half throttle. I would put the conversion/experimental engine on a more forgiving aircraft until the engine is well known.
By tuning and modifications, do you mean carb ajustments? What modifing can be done other than elect. ignition?
Most change the muffler, which almost always increases RPM and power. Many change the carburetor size. Some change the port timing especially the exhaust. Some change the intake port timing.
Bill